ISIL suicide attackers have killed dozens of people, including Syrian refugees and Kurdish security personnel, with car bombs in northeastern Syria, according to multiple sources.

The Syrian Observatory for Human Rights (SOHR) said Thursday's attack took place at Abu Fas, near the border of Deir Az Zor and Hasakah provinces, and the dead included refugees, as well as members of the Kurdish Asayish security force.

ISIL, or the Islamic State of Iraq and the Levant group, has lost large expanses of its territory in both Syria and Iraq this year and is falling back on the towns and villages of the Euphrates valley southeast of Deir Az Zor.

The US-backed Syrian Democratic Forces (SDF) alliance of Kurdish and Arab paramilitaries is pressing it from the north and a rival offensive by the Syrian army, supported by allies including Iran and Russia, is attacking it from the west.

After the blast, "the civilians escaped towards desert areas where mines exploded and the toll rose."

Much of Hasakah province and Hasakah city are under the control of a Kurdish "autonomous administration", with smaller parts of both controlled by the central government.

Abu Fas is where Kurdish authorities gather people displaced by conflict before allowing them to enter camps where they can shelter, the SOHR said.

Earlier on Thursday, the SOHR, which relies on a network of sources inside Syria, said government forces had retaken four neighbourhoods in the town of Mayadeen in Deir Az Zor.

The state news agency SANA confirmed that troops had re-entered Mayadeen.

Last week, ISIL, also known as ISIS, succeeded in expelling Syrian forces from Mayadeen, two days after they entered the town.

A Syrian army source recently described Mayadeen as the "military capital" of ISIL in Deir Az Zor.

In a separate development on Thursday, Syrian armed groups reached a ceasefire agreement for southern Damascus during a meeting in Cairo, according to Egyptian state media.

The deal includes opening main crossings and halting forced displacement of people living in the opposition-held district of Eastern Ghouta, a report in the state-run Al Ahram newspaper said.

It quoted Mohamed Alloush, the political head of Jaish al-Islam, as saying that Egypt had pledged to help break the siege on Eastern Ghouta using diplomatic means and allow in aid "in sufficient quantities to alleviate the suffering in the region".

Eastern Ghouta, near Damascus, is one of the last strongholds of fighters battling President Bashar al-Assad's forces and was the scene of a chemical weapon attack that killed hundreds in August 2013.